1. Field of the Invention
The present invention relates to an automatic gain control circuit, and more particularly, to an automatic gain control circuit including a power detector.
2. Background of the Related Art
FIG. 1 is a block diagram of a conventional automatic gain control circuit 100 including an RF received signal strength indicator (RF RSSI). Referring to FIG. 1, the automatic gain control circuit 100 includes a receiving unit 110 and a gain control unit 120.
The receiving unit 110 includes a low noise amplifier (LNA) 111, a variable gain amplifier (VGA) 112, an image-rejection mixer (IRM) 113, a low pass filter (LPF) 114, and an intermediate frequency variable gain amplifier (IF VGA) 115. The gain control unit 120 includes a gain controller 121, an RF received signal strength indicator (RF RSSI) 122, and a comparator 123.
The LNA 111 of the receiving unit 110 amplifies a received signal while maximally restraining amplification of a noise of the received signal. The VGA 112 of the receiving unit 110 amplifies the signal amplified by the LNA 111 to a signal with improved linearity within a predetermined range.
The signal amplified by the VGA 112 passes through the IRM 113 having an image rejection down-conversion function such that an image frequency is rejected to remove parasitic frequencies. That is, the IRM 113 of the receiving unit 113 separates an RF stage from an IF stage to secure stability of the receiving unit 110.
The LPF 114 of the receiving unit 110 is constructed such that it filters only a specific low band. The signal filtered by the LPF 114 is amplified by the IF VGA 115. Here, a weak received signal cannot be sufficiently amplified only with the LNA 111 of the receiving unit 110. Thus, the weak received signal is gain-controlled and amplified using the IF VGA 115 for accurate power control.
When the received signal passes through all the components of the receiving unit 110, it is amplified with its image frequency rejected and output as an intermediate frequency.
The gain controller 121 of the gain control unit 120 outputs control signals for respectively controlling the gains of the LNA 111, VGA 112 and IRM 113. The RF RSSI 122 of the gain control unit 120 measures the strength of the signal output from the LPF 114 of the receiving unit 110. The comparator 123 of the gain control unit 120 sends a gain compensation value based on the signal strength measured by the RF RSSI 122 to the gain controller 121.
Consequently, in the RF automatic gain control (AGC) loop, input signal power is detected from the output signal of the low pass filter. Thus, only the power of a desired channel is detected to control the gain of the gain stage of the RF AGC loop. Here, the RF AGC loop is constructed of the LNA 111, VGA 112, IRM 113, LPF 114, RF RSSI 122, comparator 123 and gain controller 121.
When a weak signal is input to the desired channel and a strong noise signal is input to an undesired channel, the gain stage of the RF automatic gain control loop becomes smaller than a predetermined gain. Here, though the RF AGC loop attempts to increase the gain, the strength of disturbance signal is increased to further reduce the gain. As a result, the RF automatic gain control loop cannot be normally operated.